1. Field of the Invention
This invention relates to microwave radiation and more particularly to a radiator suitable for use with either a monopulse radar or a communication tracking system where the radiator is mounted within a limited space.
2. Description of the Prior Art
When a military aircraft is being pursued by a vehicle, such as a missile or an enemy aircraft, survival of the military aircraft usually depends upon detecting the pursuing vehicle. Typically, the military aircraft has a monopulse radar for detecting the pursuing vehicle.
The radar includes an antenna mounted near the stabilizer portion of the tail of the military aircraft. Because the antenna is mounted near the stabilizer, the radar scans a spatial region aft of the military aircraft. The antenna includes a plurality of difference channel radiating horns disposed about a sum channel radiating horn.
The antenna transmits a beam that combines radiation from the sum channel horn and with radiation from a selected one of the difference channel horns. Since the difference channel horns are disposed about the sum channel horn, the direction of the beam is related to the disposition and phase of excitation of the selected difference channel horn. When the difference channel horns radiate sequentially, the radiation from the sum channel horn and the difference channel horns combine to cause the maximum of the beam to conically scan the spatial region.
When the radar operates a low frequencies, the size of such an antenna makes it difficult to mount near the stabilizer. The size may conceptually be reduced by combining the sum and difference channel horns into a single multimode horn. Furthermore, since the wavelength of an electromagnetic wave in a medium is inversely related to the dielectric constant of the medium, an antenna of reduced size may comprise a single horn loaded with a material that has a high dielectric constant. However, matching the antenna of reduced size to free space is difficult when there is a large difference between the dielectric constants of the material and free space.